The output signals generated by a circuit arrangement of this type are used for controlling a brushless electric motor. Such an electric motor has a number of winding phases or phases that are usually arranged along the circumferential direction of a stator. A rotating magnetic field is generated by means of controlled energization of the winding phases, said field driving a rotor, which generally has permanent magnets. Electric motors comprising three winding phases which are arranged in a star connection and can thus be energized via three motor terminals are known, in particular. The characteristic variables of the electric motor, such as, in particular, rotational speed and torque, can be controlled by way of the frequency, phase, intensity and duration of the winding currents, which are controlled by way of the voltages applied to the winding phases.
The duration of the energization for each individual winding phase or the duration of a corresponding voltage supply, for example for each individual motor terminal, is coded by means of the commutation signal. The commutation signal can be introduced into a control in such a way that an energization of the assigned winding phase or a voltage supply of the assigned motor terminal can be effected only given a corresponding level of the signal. In the case of a digital commutation signal, the two levels HIGH or LOW are used for this purpose.
The PWM signal is used for the variation of the voltage present at the respective winding phase during the energization or commutation phase, or for the variation of the potential present at the assigned motor terminal. In this case, a PWM or Pulse Width Modulation signal prescribes the switch-on and switch-off time of a rectangular signal with a fixed fundamental frequency. In this case, for a sluggish load, the voltage present on average can be varied by way of the duration of the switch-on time of the rectangular signal within a PWM clock cycle. If the rectangular signal is switched on for example for only half of the total available time, then the load sees on average only half of the voltage present during the switch-on time.
In a conventional circuitry interconnection, for the driving of the respective winding phases or motor terminals of a brushless electric motor, the PWM signal is logically combined with the commutation signal by means of a logic component in the manner of a gate circuit. If an OR gate, for example, is used as the logic component, then a signal which corresponds to the PWM signal when the commutation signal is simultaneously at the level LOW is available at the output of the OR gate for control purposes. The assigned motor terminal is supplied with a PWM-clocked voltage signal according to the output signal. If the commutation signal is switched to the level HIGH, then the output signal of the OR gate is at the level HIGH in constant fashion. The motor terminal is supplied with a constant voltage signal, in particular is pulled to ground or to a low reference potential. The situation is correspondingly reversed in the case where an AND gate is used as the logic component.
Such a circuit arrangement in accordance with the prior art makes it possible to use, for the driving of a brushless electric motor, a cost-effective control component having a small number of PWM contacts or just one PWM contact. The PWM signal of a single PWM contact is used by means of corresponding logic components for the modulation of a plurality of commutation signals.
The degrees of freedom with regard to the possibilities for driving the electric motor are disadvantageously restricted by such a circuitry interconnection. If the driving of the electric motor takes place for example by means of a converter which is fed by an intermediate circuit and which has a number of controllable switching elements for the clocked changeover between the two potentials of the intermediate circuit, then for a winding phase the so-called HIGHSIDE switching element, that is to say that switching element which switches the motor feed line to the high potential of the intermediate circuit, cannot be driven independently of the LOWSIDE switching element, which switches the motor feed line to the low potential of the intermediate circuit. This is because only a single PWM signal is available for both switching elements. It is likewise not possible to drive individual winding phases independently in the case of an overlap of the commutation phases of said individual winding phases if only one PWM signal is available.